The primitive endoderm (PrE, also named hypoblast), a predominantly extraembryonic epithelium that arises from the inner cell mass (ICM) of the mammalian pre-implantation blastocyst, plays a fundamental role in embryonic development, giving rise to the yolk sac, establishing the anterior–posterior axis and contributing to the gut. PrE is specified from the ICM at the same time as the epiblast (Epi) that will form the embryo proper. While in vitro cell lines resembling the pluripotent Epi have been derived from a variety of conditions, only one model system currently exists for the PrE, naïve extraembryonic endoderm (nEnd). As a result, considerably more is known about the gene regulatory networks and signalling requirements of pluripotent stem cells than nEnd. In this review, we describe the ontogeny and differentiation of the PrE or hypoblast in mouse and primate and then discuss in vitro cell culture models for different extraembryonic endodermal cell types.

原始内胚层（PrE，也称为下胚层）是一种主要由哺乳动物着床前囊胚的内细胞团 (ICM) 产生的胚外上皮，在胚胎发育中发挥着重要作用，产生卵黄囊，建立前后轴并有助于肠道。PrE 与将形成胚胎的外胚层 (Epi) 同时从 ICM 中指定。虽然类似多能 Epi 的体外细胞系源自多种条件，但目前仅存在一种 PrE 模型系统，即幼稚胚外内胚层 (nEnd)。因此，人们对多能干细胞的基因调控网络和信号转导要求的了解远多于 nEnd。在这篇综述中，我们描述了小鼠和灵长类动物中 PrE 或下胚层的个体发育和分化，然后讨论了不同胚胎外内胚层细胞类型的体外细胞培养模型。